\documentclass{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}$$ t_{\alpha/2} = \frac{|\overline{X_1}  - \overline{X_2}|}{\sqrt{\frac{S^2_1}{n-1} + \frac{S^2_2}{n-1}}}$$\end{equation*}
\end{document}
